There are many and diverse situations where threaded connections are used to interconnect two members which are subjected, after the connection has been made, to loads along the axis of the threads. Usually the magnitude of the applied axial load is known with more or less precision as a result of analyses made in conjunction with the design of the connection. The thread characteristics are specified with reference to and in consideration of the load which the connection is designed to carry or withstand; such load is often referred to as the "design load".
Conventional threaded connections use threads of constant pitch in which the threads have an appropriate one of several standard profiles or a specially designed profile. Such threads have the common feature that when threaded together to a "hand-tight" or a lightly loaded condition, all the engaged threads are in contact with each other. However, as axial loads are applied thereafter to the connection, the two members involved in the connection strain (deform) in response to the applied load, thus causing the increasing load to be carried by progressively less and less of the length of the engaged threads. Ultimately, the applied load may reach a magnitude sufficient to cause the threads actually carrying the load to begin to fail in shear, a phenomenon commonly called "stripping". Once stripping begins and the applied load is not reduced, it proceeds progressively along the entire threaded connection causing more and more of the thread length to fail in shear until all of the threads in the connection are stripped and the two members in the connection separate from each other.
As an example, consider the case of a bolted-down access door into a pressure vessel. The cover is connected to the exterior of the vessel by use of externally threaded studs which pass from the vessel through corresponding holes in a flange of the door into engagement with internally threaded nuts. Each stud and nut set forms the two members involved in a threaded connection. Each threaded connection is initially made up to the desired tightness by screwing the nut down on the stud against the door flange. At this point, the load faces of the nut threads (the faces of the nut threads which face away from the door) are essentially fully engaged with the stud thread load faces (the faces of the stud threads which face toward the door and which are engaged with the nut thread load faces). The end of the nut adjacent the door is compressed against the door flange; this compression produces tension in the stud. Assume that no pressure relief mechanism is provided for the pressure vessel so that pressure in the vessel can increase above the desired design pressure. Pressure in the vessel begins to increase. This pressure increase is applied to the door, urging the door away from the vessel along the door mounting studs. Movement of the door away from the vessel is resisted by the nuts which are further compressed, and by the studs which experience increased tension. This compression is a maximum at the end of the nut bearing against the door, and the tension in the stud is manifested in full at the stud thread which is engaged with those nut threads closest to the door. The nut and the stud are made of metal. The compressive load applied by the door to the nut causes the nut metal to strain (deform) in compression, i.e., to shorten axially; such strain is maximum in the nut adjacent the door and decreases progressively proceeding along the nut away from the door. The stud tension causes the stud to strain in tension, i.e., to elongate axially; such strain is uniform in the stud along its length not engaged with the nut and decreases throughout that portion of its length engaged by the nut. The important point is that the material of the nut immediately adjacent the door, and the coaxially adjacent material of the stud are urged by the applied load (the pressure in the vessel) to move in opposite directions along the thread axis, and such opposite movement is resisted by the threads which are loaded in shear. The thread shear loading is maximum immediately adjacent the door-to-nut interface, and decreases proceeding along the threads away from such interface. If the pressure-related load on the threaded connection increases above the design pressure level (i.e., the design load level for the threaded connection), the shear strength in the threads will be exceeded at some point, if the stud does not first fall in tension. The threads will begin to shear (strip) immediately adjacent the door-to-nut interface. The loads applied to the connection will be transferred to the remaining threads which will similarly be overloaded so as to fail in shear. The result is that the threads progressively shear off until the nut separates from the stud and the connection fails entirely.
The situation described above could not occur if, at the time the pressure in the vessel reached design pressure, thereby presenting the design load to the threaded connection, all engaged threads in the connection were uniformly and lightly loaded, rather than nonuniformly loaded with some of the threads heavily loaded and others only very lightly loaded. If all engaged threads are uniformly and lightly loaded when the connection is subjected to the design load, the connection would be much better able to withstand applied loads substantially in excess of the design load. Such threaded connections are presently not known, but a need for such connections exists for use in many different situations.